The present invention relates to processes for recovery of bitumen tar sands, and, in particular, to a method for direct coking of tar sands without requiring a prior separation of the bitumen.
The rapid escalation in petroleum prices, along with the uncertainties related to the continuous supply of oil, provides motivation for countries to review their inventory of fossil fuels with a view to developing resources of lower quality oil supplies such as oil shales, tar sands, and heavy oils. Generally, such resources are of lower grade than conventional petroleum because they contain a greater proportion of mineral matter, a lower hydrogen content, and a higher proportion of foreign atoms, such as oxygen, nitrogen, and sulphur, which have to be removed to make acceptable fuels for commerce. These factors increase the capital requirements necessary to produce acceptable products, whether it be fuel or petrochemical.
Tar sands, also known as oil sands or bituminous sands, are sand deposits impregnated with a dense, vitreous petroleum-like material generally termed bitumen. Tar sands are found throughout the world and the largest known deposites are in the province of Alberta, Canada and Eastern Venezuela, although deposits in the lower continental United States are also believed to be quite sizeable.
The tar sands are primarily silica, closely associated with petroleum-like material (heavy oily material) which varies from about 5 to about 21% by weight, with a typical content of 13 weight percent comprising the sand. The oil is quite heavy, 6.degree. to 8.degree. API gravity and contains typically 4.5% sulfur and about 38% of aromatics. Tar sands also include clay and silt in quantities of from 1 to 50 weight percent, and water in quantities of 1-10% by weight.
Bitumen can be separated from the sand by a variety of methods, including first in situ thermal, emulsion-steam drive, and even atomic explosion mining; followed by processing of various types such as direct coking, anhydrous solvent extraction, cold water separation, hot water separation, and the like; and any of these followed by possibly various methods of upgrading the separated bitumen to a more salable product generally described as synthetic crude oil.
Despite the fact that the existence of tar sand deposits has been known for years, and that the bitumen can be separated for crude oil production, nevertheless, as a matter of fact, separating the bitumen on a practical, economical basis has met with little success.
A thermal method of recovering bitumen by direct retorting has been studied since 1940. In direct retorting, raw tar sand can be contacted with spent sand and fluidized by reactor off-gas at a temperature around 900.degree. F. The volatile products are flashed off, while coke is deposited via thermal cracking. The coked sand can then be burned off in a separate unit at 1200.degree.-1400.degree. F. and recirculated.
Early work on direct retorting of tar sand is reported by P. E. Gishler and W. S. Peterson in "Oil from Alberta Bituminous Sand" in Petroleum Engineer, Vol. 23, Issue 23, pp. c 66-c 76 (1951), and "The Fluidize Solids Technique Applied to Alberta Oil Sands Problem" in Proceedings of the Alabasca Oil Sands Conference, Edmunton, Alterta, pp. 207-236 (1951), and by R. W. Ramuler in "The Production of Synthetic Crude Oil from Oil Sand by Application of the Lurgi-Rhurgas Process" in the Canadian Journal of Chemical Engineering, Vol. 48, pp. 552-560 (1970). The technology of retorting tar sand in general is known as and is referred to herein as, direct coking.
One of the known processes for direct coking of tar sands is the Lurgi-Rhurgas, which is a dry distillation process wherein mined tar sand is fed into a mixture from a feed bin and is contacted with hot recirculated sand at 1200.degree. F. (The sand is heated in a lift heater by burning off the coke that is deposited on the sand in the mixer.) A final retorting temperature of 900.degree. to 1,000.degree. F. is attained whereby the bitumen is broken down into a liquid, vapor, and entrained solids. The product stream is sent to a cyclone to remove particulate, and then to scrubbers to recover the light gases and product liquids. The hot sand from the mixer drops to a surge hopper where it is divided into two streams, one stream being recirculated to the mixer through the lift heater and the other stream being sent to an air pre-heater. While the overall bitumen recovery in the Lurgi-Rhurgas dry distillation process is estimated at 70%, this system suffers from several disadvantages, including severe heat loss in spent sand, small unit throughput and poor scale-up economics, expensive construction and replication of retorting equipment, and inefficient methods for preparation and handling of tar sand feed.
Another process for direct coking is the Taciuk process which is similar to the Lurgi-Rhurgas process except for the equipment. In the Taciuk process, the bitumen is retorted in a rotating kiln, rather than the mixer and surge hopper of the Lurgi-Rhurgas process. The Taciuk process suffers a disadvantage of having limited scale-up possibilities particularly because of seal problems associated with a large rotating kiln.
It is, therefore, an object of the present invention to overcome problems generally known in the art of direct coking, as set forth above, and in general, to make direct coking a commercially viable process by, inter alia, eliminating the need for hot screw feeds or rotating kilns.